1. Field of the Invention
This invention relates generally to group III-nitride crystal growth and more specifically to gallium nitride crystal growth.
2. Description of Related Art
Because they can emit light from the ultraviolet to the visible spectral range, group-III nitrides (III-N) are the most promising materials for optoelectronic light sources (S. Nakamura, et al., Jpn. J. Appl. Phys. 34, L797 (1995); S. Nakamura, et al. Jpn. J. Appl. Phys 35, L217 (1996)). They also hold promise for high frequency power devices (M. A. Kahn, et al. Appl. Phys. Lett. 63, 1214 (1993). Usually, III-N films, for example GaN, are grown at temperatures that are below half the melting point temperature to prevent chemical decomposition during the growth process. In addition to semiconductor materials, III-N films also comprise ceramics which have many uses when deposited as thin films. This is described in, "A study of the physical properties and electrochemical behavior of AlN films" F. Vacandio, Y. Massiani, P. Gravier, Surface Coating and Technology 92, 1997, 221.
Two primary methods are currently used to grow III-N films such as, for example, GaN films: Metal Organic Chemical Vapor Deposition (MOCVD) and Molecular Beam Epitaxy (MBE). MOCVD is performed at high temperatures, about 1300 K, which leads to relatively high surface diffusion of ad-atoms. Each atom that is newly deposited on a substrate surface is referred to as an "ad-atom". Because there is a direct relationship between surface diffusion and formation of large crystal grains, MOCVD has in the past resulted in larger grain formation than MBE. Conventional MOCVD yields grain diameters in the range between about 40 .mu.m to about 50 .mu.m while conventional MBE yields grain diameters between about 0.5 .mu.m and about 1 .mu.m. Large grain size is desirable because it decreases the boundaries that charge carriers encounter, which in turn increases carrier mobility. A full discussion of the importance of carrier mobility and grain size can be found in "Semiconductor Devices, Physics and Technology" S. M. Sze, New York Wiley, 1985. Ideally the entire thin film grown would be of a single crystal. Instead, a number of crystals are formed which grow into one another to form the thin film. It is important to minimize the discontinuities at the boundaries between these crystal growing regions and this is done both by growing larger crystal regions so that there are fewer boundaries and by maintaining high degree of common crystal lattice orientation between the crystal regions. When the lattice orientation is maintained sufficiently, these crystal regions are referred to as `grains` in the larger crystal thin film.
Although the MOCVD process results in large grain sizes, there are several drawbacks to the process. Suitable metal organic precursors must be available to create and dope the desired III-N thin film. The restriction in starting materials limits the types of end product that can be formed. In addition, the high temperatures at which MOCVD films are formed are close enough to thermal equilibrium to limit variation in the percent of various materials that make up the final film.
In contrast, the MBE growth process typically takes place at about 1000 K. Forming a film at temperatures that are farther away from thermal equilibrium of the III-N starting materials, allows higher concentrations of dopant to be incorporated into the final structure. In addition, the lower the film formation temperature is, the more possible it is to manipulate the concentration of component parts of the final film. Ad-atoms having less thermal energy are less likely to exclude a substitute atom from the forming lattice. Thus, forming a film at a lower temperature can help to increase doping levels or to grow high quality structures comprising added elements such as aluminum and/or indium. For example, using the relatively low crystal growth temperatures of MBE, compounds such as Ga.sub.X Al.sub.1-X N and Ga.sub.Y In.sub.1-Y N can be formed, where x represents the relative percent Ga with respect to Al and y represents the relative percent Ga with respect to In. It can be seen that when x=0, the first compound becomes aluminum nitride (AlN), and when y=0, the second compound becomes indium nitride (InN). Alternatively, when x and y=l, the compounds become GaN. The drawback to MBE films grown at relatively low temperatures, for example about 30% to about 40% of a compound's melting point temperature (about 1000 K for GaN), is that the MBE films frequently exhibit a three-dimensional, instead of a two-dimensional, crystal growth and grain sizes that are limited by temperature and by strain. Films made using conventional MBE technology therefore have exhibited reduced carrier mobility. Excess strain originating from mismatched substrate and film lattice structures can be optimized by growth of suitable buffer layers on the substrate before a III-N film is grown. But it is not possible to increase the temperature at which MBE takes place because the differential vapor pressures of the nitrogen atoms alters the film composition under the vacuum in which MBE takes place.
It would be very desirable to have a method of growing large two-dimensional crystals of III-N compounds at low temperatures, utilizing flexible MBE technology.